Recording systems in which a laser beam is used, which is to say laser beam recording systems, exist for electrophotographic recording purposes. In these systems a recording is made by focusing the laser light emerging from a laser with an f.theta. lens, forming a scanning image on a photosensitive body by means of a polygonal mirror and developing and transcribing this image as required.
The development of photosensitive materials which are sensitive to the wavelength region about 700 nm has become desirable in recent years as a result of the development of low output semiconductor lasers (in practice, lasers which have an output of some 5 to 25 mW). The electrophotographic photosensitive materials used with semiconductor lasers of this type require various properties different from those of the conventional electrophotographic photosensitive materials. Thus, an adequate sensitivity to radiation from the near infrared region into the infrared region and good dark charge-retaining properties are especially important.
The use of various spectral sensitizing dyes in electrophotographic photosensitive layers comprising photoconductor-resin dispersion based photoconductive composition is known. For example, dyes for spectral sensitization to red light and infrared radiation have been disclosed in U.S. Pat. Nos. 3,619,154 and 3,682,630, but these dyes are easily degraded particularly during the storage of the dye or during the manufacture and storage of the electrophotographic photosensitive layer and so their performance is poor and this is greatly disadvantageous in practice. Harazaki et al. have stated that the sensitizing dyes for red light and infrared radiation are more unstable that the sensitizing dyes for light of shorter wavelengths (visible light) (Industrial Chemistry Journal (Japanese), Vol. 66, No. 2, page 26 (1963)).
Furthermore, cyanine dyes for spectral sensitization when zinc oxide is used as a photoconductor have been disclosed in Japanese Patent Application (OPI) Nos. 58554/83, 42055/83 and 59453/83 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). However, these cyanine dyes do not extend into the near infrared and infrared wavelength regions nor are they sufficiently stable in the photosensitive body. Thus, in either case it is impossible to achieve a satisfactory level of sensitivity.
Electrophotographic photosensitive materials comprising photoconductive compositions which contain heptamethine cyanine dyes which have a 3,3-dialkylindole ring or a 3,3-dialkylbenzo[e]indole ring at both ends are disclosed in U.S. Pat. No. 4,362,800. The sensitized range of these photosensitive materials extends to 750 nm and above and they also have good stability.
However, there is a disadvantage in that these prior art electrophotographic photosensitive materials have inadequate dark charge-retaining properties. As mentioned above, the situation when a semiconductor laser is used as a light source differs from that in conventional full-surface exposure systems in which visible light is used, in that a scanning exposure system is used and the period beginning from the time at which the photosensitive material is charged until the exposure is completed is longer. The unexposed parts must retain their charge satisfactorily during this time. Thus, dark charge-retaining properties are a very important feature for electrophotographic photosensitive materials for use with scanning exposures.
Furthermore, the light sources have a low output and so a sufficiently high sensitivity in the near infrared to infrared region is required. The prior art electrophotographic photosensitive materials mentioned above are also inadequate in this respect.